I usually give them some reviews to read and, invariably, the experts who write the reviews will say that sex is a great unsolved mystery. If the experts think that this is a mystery then why do so many people think they have the answer?
Here's what Douglas Futuyma says in the 2nd edition of Evolution (p. 340).
The recency of most parthenogenic lineages suggests that sex reduces the risk of extinction. If this were the reason for its prevalence, sex might be one of the few characteristics of organisms that has evolved by group selection. But recombination and sex also have serious disadvantages. One is that recombination destroys adaptive combinations of genes.... In general, asexual reproduction preserves adaptive combinations of genes, whereas sexual reproduction breaks them down and reduces linkage disequilibrium between them. An allele that promotes recombination, perhaps by increasing the rate of crossing over, may decline in frequency if it is associated with gene combinations that reduce fitness.Note the phrase, "... one of the most difficult puzzles in biology," How many readers think that sex and/or recombination is a great mystery? Not many, I'll bet. I'm guessing that most of you are quite satisfied with the answer that sex confers evolutionary advantage because it increases diversity (combinations of alleles).
Sexual reproduction has a second disadvantage that is great enough to make its existence one of the most difficult puzzles in biology. This disadvantage is the cost of sex. Imagine two genotypes of females with equal fecundity, one sexual and one asexual. In many sexual species, only half of all offspring are female. However, all the offspring of an asexual female are female (because they inherit their mother’s sex-determining genes). If sexual and asexual females have the same fecundity, then a sexual female will have only half as many grandchildren as an asexual female. Therefore, the rate of increase of an asexual genotype is approximately twice as great as that every sexual genotype (all else being equal), so an asexual mutant allele would very rapidly be fixed if it occurred in a sexual population.
You all should read what Joe Felsenstein is saying on What did Joe Felsenstein say about sex?. You don't have to agree with him, or with Futuyma, but if you are a teacher you should make sure that students understand the controversy. The mystery of sex has not been solved.
... sexual reproduction breaks apart favorable combinations of genes built by past selection. To hammer this point home, consider an analogy. Imagine entering a poker hall after a game has been played. If you were to offer the winners (holding, say, a 3♣, 4♣, 5♣, 6♣, 7♣ straight at one table, a three‐queen hand Q♦, Q♥, Q♠, 2♥, 8♦ at another, etc.) the opportunity to keep their hands or to shuffle their cards with those of another, everybody would hold his or her cards. Winning hands—those that have “survived” previous rounds—have cards that work well with one another. Shuffling these cards together produces descendant hands with no guarantee of success (creating, e.g., a lousy hand of 3♣, 4♣, Q♠, 2♥, 8♦). In all card games of interest, it is not enough to know the suit and number of each card in isolation; rather, the interactions among cards are what determine whether the card is in a winning hand or a losing hand. Similarly, genes do not work in isolation; the interactions among an individual’s genes in the context of its environment are what determines whether that individual will successfully survive to reproduce or fail. Sexually mixing one's genes with those of another destroys the network of alleles that worked well in the parent, creating a new network that may or may not function....She goes on to discuss "I. Sex Need Not Increase Variation" and "II. Generating Variation through Sex Often Reduces Fitness."
Given the costs of sex and the widespread potential for asexual reproduction, why do so many species reproduce sexually? This question has been called the paradox of sex. Most biologists would answer that sex and recombination have evolved because they generate variation needed by selection. Indeed, this is one of the oldest explanations for sex, attributed to August Weismann (1889, p. 279):
Sexual reproduction can also increase the differences between individuals …. Such differences afford the material by means of which natural selection is able to increase or weaken each character according to the needs of the species.That sex evolved to generate variation may very well be correct, but there are two holes in the argument that make it a much less obvious answer than it would at first seem. Because these holes are not widely appreciated, even among evolutionary biologists, they deserve some attention.
Isn't it interesting that an expert on the subject would use a title like "The evolutionary enigma of sex"?
Otto, S.P. (2009) The evolutionary enigma of sex. The American Naturalist 174, S1-S14. [doi: 10.1086/599084[